Polymer composition for peroxide bleaching of cellulosic fibre material and processes for peroxide bleaching

Abstract

The present invention relates to a polymer composition for peroxide bleaching of a cellulosic fiber material, said composition being in the form of a stable aqueous polymer solution obtained by bringing in an aqueous medium a first polymer (A) comprising a partly or totally acidic homopolymer of acrylic acid, methacrylic acid or maleic acid, or a copolymer of acrylic acid and / or methacrylic acid with an unsaturated dicarboxylic acid, into contact with a second polymer (B) comprising a polylactone of a poly-alfa-hydroxyacrylic acid in solid form or as a moist powder or as a slurry, said polymer solution having a pH of at most 8, The invention also relates to peroxide bleaching processes.

Description

FIELD OF THE INVENTION[0001]The present invention relates to a stable polymer composition for peroxide bleaching of a cellulosic fibre material. The stable polymer composition can be used in alkaline peroxide bleaching of mechanical, chemical, chemi-mechanical and de-inked pulps and in deinking of recycled fibers without using silicate as a stabilizer. The present invention also relates to a process for bleaching a cellulosic fibre material with a peroxide compound in an aqueous alkaline medium by using said stable polymer composition.DESCRIPTION OF THE RELATED ART[0002]Alkaline silicate solutions normally called water glass have been used in stabilizing hydrogen peroxide solutions, which are used in alkaline peroxide bleaching of mechanical pulps. Nowadays it is more common not to make a pre-prepared bleaching liquor, but water glass is fed separately, when mechanical pulps are bleached with peroxide and alkali.[0003]Water glass is used alone or together with peroxide in de-inking ...

AI Technical Summary

Benefits of technology

[0025]This method is a very efficient way to make a polymer composition, since the polylactone can easily be made at high solids content, which is not easy for the sodium salt of PHAA and which thus can be transported more cheaply than e.g. the alkali salt of PHAA. No neutralization agent is needed for commercial use of this composition and the composition can be made at higher solids content than the sodium salt of PHAA.

[0026]An extra advantage with the use of this slightly acidic polymer composition is that the alkaline hydrogen peroxide process can be more easily adjusted for the right pH by adding alkali in the bleaching process than by using the alkali salts, such as e.g. sodium salt of PHAA and the conventional salts of polycarboxylic acids, which need a more sophisticated control to reach the optimal pH at the alkaline peroxide bleaching.

[0051]The bleaching of mechanical pulps can be carried out at a temperature of 30-90° C., preferably at a temperature of 50-90° C. The bleaching can be carried out at a consistency of choice, but it is most preferably to carry out the bleaching at a high consistency, i.e. about 30% or higher. Bleaching can also be carried in two stages with a dewatering stage between the stages. The stages can be carried out at a consistency of choice, but it is most preferably to use medium consistency in the first stage and a high consistency in the second stage. This makes it possible to remove the detrimental substances efficiently. A chelating agent treatment and dewatering can precede the bleaching stage and thus improve the bleaching performance. The consistency of this pretreatment is preferably around 10% in order to ensure an efficient metal removal. The pH should be from 3 to 7, preferably 4 to 6.5 and most preferably from 4.5 to 6, if conventional chelating agents are used. Conventional chelating agents, such as polyaminopolycarboxylates, such as EDTA or DTPA or the corresponding phosphonic acids such as EDTMPA and DTPMPA can be used as the chelating agents.

Problems solved by technology

The use of water glass in alkaline peroxide bleaching of chemical pulps has been published, but the method cannot be utilized in full scale, since the silicate can cause very severe precipitation problems.

Another disadvantage with water glass is that when the bleaching liquors are recycled and ultimately fed into the recovery boiler, where the so-called black liquor from the cooking process after concentration is burned, the silicate will cause severe scaling and thus decrease the heat transfer in the recovery boiler, which in worst case can cause an explosion of the recovery boiler.

Further the use of the silicate can cause highly dusting.

If the silicates, e.g. in form of the water carry-over, will enter the paper making process, they will disturb the papermaking process, e.g. by precipitating on hot surface, causing holes in the paper reel etc.

The copper ion is also very detrimental for alkaline hydrogen peroxide, but normally it can enter the process only via used process waters.

The corresponding phosphonates, EDTMPA and DTPMPA can be also used, but they are much more expensive than the polyaminopolycarboxylates.

They have also the disadvantage that they contain phosphorus, which is not a wanted component, when the environmental regulations are becoming stricter and stricter.

There have been suggestions to use phosphonates, but they should be used in quite high dosages and the phosphorus problem in the waste waters would still remain.

Since it is known that hydrogen peroxide will very easily decompose in alkaline solutions, a ready made bleaching mixture is not a very feasible choice when bleaching pulp with hydrogen peroxide in an alkaline milieu.

It has been found that the alkaline earth metal ions can be very detrimental for PHAA.

If PHAA is used alone it will bind calcium and magnesium ions and render PHAA less effective due to the content of alkaline earth metal ions.

This means that a smaller amount of the effective and expensive component, i.e. PHAA, could be used, when the product made from the polylactone and the polycarboxylate was used than when using PHAA made from the polylactone alone.